Electric resistance heating device



March 6, 1956 P. EISLER 2,737,571

ELECTRIC RESISTANCE HEATING DEVICE Filed July 29, 1955 5 SheetsSheet l riiaiam-la mn fimwif A ftorney March 6, 1956 P. EISLER ELECTRIC RESISTANCE HEATING DEVICE 3 Sheets-Sheet 2 Filed July 29, 1953 Inugntor //M M flew 5M Attorney March 6, 1956 p ElsLER ELECTRIC RESISTANCE HEATING DEVICE Filed July 29, 1953 3 Sheets-Sheet IGIB Inventor United States Patent 9 ELECTRIC RESISTANCE HEATING DEVICE Paul Eisler, London, England, assignor to Teclmcgraph Printed Circuits Limited, London, England, a company of Great Britain Application July 29, 1953, Serial No. 371,039

Claims priority, application Great Britain August 8, 1952 13 Claims. (Cl. 219-38) This invention relates to electric resistance heating devices. For many purposes it is preferable to have a large heating surface which is at a comparatively modest temperature rather than a small heating surface which is at a high temperature, and it is an object of the present invention to provide an electric resistance heating device which satisfies this requirement.

A particular application of the invention is to the heating of vessels such as water tanks, where the heating surface may constitute at least a part of the wall of the vessel. it also has other applications, for instance to immersion heaters, to space heating devices whether of the radiation or convection type, to cooking vessels, and to booster heaters for liquid-filled radiators.

According to the invention an electric heating device comprises a resistance element of large surface area to cross-sectional area ratio, contained within and spaced from the walls of a jacket at least a part of which is heat conductive and constitutes the heating surface, which jacket is filled with a liquid which is an electric insulator and which can transmit heat from the resistance element to the walls of the jacket.

I By providing a resistance element having a large surface area to cross-sectional area ratio, a large heat transfor surface from the resistance element to the liquid is achieved. There is also a large area of contact between the liquid and the walls of the jacket. In consequence, for the transfer of a given quantity of heat, the resistance element can be maintained at a lower temperature than would otherwise be necessary. Not only is this advantageous for the efficiency of the heat transfer but also it increases the thermal safety factor, and widens the range of suitable liquids, since the liquid must be one which is stable at the maximum temperature attained by the resistance element.

The resistance element may consist of lengths of fine resistance wire connected in parallel, the total length of wire employed being very large in order to afford a large surrace area, but a preferred form of element comprises a pattern of conductors formed of thin metal foil. Such an element can readily be made by mass production methods, for instance by stamping from a continuous strip of metal foil or by one of the so-called printed circuit techniques which have been developed in recent years for forming conductive patterns f metal foil. Conductive paths of almost any desired width and length (within theliinits set by the dimensions of the original strip) can be formed in a simple manner merely by cutting or otherwise forming slits in the foil at suitable places. The design should be such that the length of each slit is short in relation to the width of the strip, the slits being disposed in staggered relationship so as to minimise weakening the strip. in particular, any slits which extend to the edges of the strip should be kept as short as possible; indeed, so far as practicable the element should designed so that a minimum number of slits extend to the edge of the strip.

Conveniently the resistance element is spaced from theice walls of the jacket by one or more layers of porous or other liquid-permeable insulating material wrapped around or laid over the resistance element. The thickness of the insulation should be sufficient to withstand effectively the highest voltage likely to be applied to the resistance element, whether in actual use or during testing, and to prevent any possibility of contact between the resistance element and the walls of the jacket, even in the event of thermal distortion of the resistance element. On the other hand the insulation should be as thin as possible, consistent with the safety requirements just mentioned, so that the length of the heat transfer path through the liquid is smal. The layer of insulation, or each layer Where there are more than one, may comprise, for instance, a sheet of porous paper, or a woven mat of glass wool or other insulating fibre, or a thin wad of felted glass wool fibres. The layers of insulating material may be sewn on to the resistance element.

The resistance element may have an elastic anchorage at least at one edge or end, to maintain it in tension.

In one form of the invention the resistance element is arranged closer to one wall of the jacket, which consti'tutes the main heating surface, than to the opposite wall, so that the heat transfer to the former wall is greater than to the latter wall.

For many purposes transformer oil is suitable as the liquid filling the jacket, but for other purposes certain silicone oils or other synthetic liquids which have been developed for heat transfer and electrical insulation purposes may be employed. To reduce the risks of spilling and leakage, the liquid may be one which is highly viscous or a gel at room temperature.

it part of the jacket wall is not to constitute the heating surface, this part should be heat-insulated to minimise heat losses. This may be done by lagging, either externally or on the inner surface of this wall. However, for cheapness and for simplicity of manufacture this part of the jacket may comprise a double wall enclosing a dead air space, so as to afford air insulation. In some instances the outer member of the double wall may be constituted by part of a casing in which the heating device is installed.

The invention may be performed in various ways, and two particular embodiments will now be specifically described by way of example with reference to the accompanying drawings, in which:

Figure l is a sectional elevation of the tub of a domestic washing machine, with one form of heating device embodying the invention in position;

Figure 2 is a section through one end of the heating device of Figure l on a larger scale;

Figure 3 is a perspective view of the heating device of Figure l partly cut away to show details of its construction;

Figure 4 is a fragmentary view of the resistance eleent, also illustrating the manner of its construction;

Figure 5 is a sectional elevation of another embodiment of the invention suitable either as a cooking vessel or as an immersion heater for use in a larger cooking vessel; and

Figure 6 is a fragmentary sectional view of part of the vessel of Figure 5, on a larger scale.

The tub of the washing machine shown in Figure l is made up of three main members, namely a front wall member (not shown), a back wall member 10 which carries the agitator 11, and an integral bottom and side wall member 12. The latter member is a wide strip of metal bent into the form of a U. the limbs of the U constituting the side walls of the tub and the base of the U constituting the bottom of the tub. The edges of the strip are bent outwardly to form shallow flanges 13 to which are welded the edges of the back and front wall members, respectively. Between the flanges 13, therefore; there is a wide'shallow channel extending down one element 1e. be shaped so that it lies well clear of the drain pipe.

side wall of the tub, around the bottom, and up the other side wall.

A heating device 14 embodying the present invention is fitted in the said shallow channel. The inner wall of the jacket of the device is constituted by the said U-shaped member 12 and serves as the heating surface. The outer wall of the jacket is formed by another U-shaped wide strip 15 the edges of which are welded or otherwise secured in a liquid-tight manner to the outwardly-facing flanges 13 on the U-shaped member 12.

In the jacket is arranged the heating device 1%. This comprises several parallel strips of thin metal foil In, a part of one of which is shown in Figure 4. Each strip of metal foil is provided with numerous rows of short transverse slits 16A and 168, the slits in adjacent rows being disposed in staggered relationship so that each strip comprises several serpentine conductive paths connected in parallel.

The slits 16A are relatively short in relation to the total width of the strip 16, and the slits 163 which extend to the edges of the strip are even shorter. In manufacture the strip is made wider than its finished width, as shown in the left-hand part of Figure 4, with marginal portions 16C closing the ends of the slots 163. This makes the strip less liable to damage during storage and handling. Before the strip is actually used the marginal portions 16C are trimmed off so as to open the slits 15B.

The strips of the resistance element are sandwiched between two layers 17 of glass cloth, which are covered by insulating porous mats 18 of glass wool on both sides. These mats are protected by outer layers 19 of glass cloth. Extra layers of glass cloth may be provided as at 20, for additional reinforcement. The insulation is conveniently secured to the metal foil resistance element by stitching. The cross-section of the resistance element 16 and its insulation 17-20 is approximately the same as the cross-section of the jacket in which the resistance element and the insulation are installed, so that the insulation locates the resistance element centrally in the jacket. The remaining spaces in the jacket are filled up to the level 21, with a liquid such as transformer oil, which penetrates all the pores in the insulation and provides a heat transfer path from the resistance element 16 to the jacket walls 12 and 15. This liquid is preferably one which, while fluid at the heating temperature, is highly viscous or a gel at room temperature.

To anchor the ends of the heating device, the insulating layers 17, 18 and 19 are fastened together, as by stitching 22, just above the upper edge of the resistance element 16, and the inner layers of glass cloth are continued upwardly and joined together to form a loop 23 through which passes a rod 24. The loop 23 containing the rod 24 is fitted into a correspondingly shaped recess in a sealing strip 25 of Neoprene or other resilient material which is compatible with the liquid contained in the jacket. These strips 25 fit firmly between a shallow groove 26 formed in the top of the wall 15, and the outer surface of the wall 12. To tension the heating device, resilient strips 27, of Neoprene or the like are sewn between the layers 17, the parts of the layers 17 between the rows of stitching 2S and 29 being left loose so that the tension is applied by the strips 27 only.

The leads 3:) to the resistance element 16 are taken in through one corner of the jacket, passing through a flexible grommet 31 which is shaped to make sealing contact with the adjacent parts.

A drain pipe 32 from the bottom of the tub passes through the jacket. A seal 33 is provided where the drain pipe passes through the outer jacket wall, to prevent escape of the filling liquid, and the length of the pipe between the inner and outer jacket walls may be insulated to prevent it from coming into contact with the resistance Alternatively, the resistance element may A 4 slit 34 is provided in the heating device to facilitate assembly.

When the tub is installed in the casing of the washing machine the side walls 35 of the casing lie fairly close to,

the outer wall 15 of the jacket, afiording a dead air space 36 which provides air insulation for the outer wall. If preferred, this space may contain lagging. Alternatively, there may be a second U-shaped strip (not shown) attached to the tub outside the outer wall 15 of the jacket so as to provide a double outer wall containing a dead air space, or which may contain lagging if preferred.

The resistance element 16 should be designed so that if through some misadventure the liquid in the heating device should leak away, or if the device should be left switched on when there is no water in the tub, the temperature of the heating element will not rise to a dangerously high value. In this way the necessity for providing temperature-sensitive safety devices may be avoided.

In a modification the resistance element may be in two or more parts which can be connected in parallel for heating the water from cold, and then connected in cries for keeping the water hot, the current consumption and the heat generated being less in the series-connected than in the parallel-connected condition.

instead of disposing the heating device on the outer surface of a wall of the tub or tank, it may, if more convenient, be disposed on the inner surface, since water or other liquid cannot penetrate the jacket. Alternatively, the heating device may be made up in the form of a fiat irnn ersion heater, which can be fitted into the tub or tank without materially interfering with the contents.

In this form the immersion heater may either be a permanent fitting or may be removable.

In the embodiment of the invention shown in Figures 5 and 6 the heating device is in the form of a dish or pan, which can be used on its own as a cooking vessel or which can be used as a wholly or partially immersed immersion heater in a larger vessel. In Figure 5 device is shown in the latter form. In this embodiment the jacket is a metal casing having an inner wall 40 and an outer wall 41. The heater is contained in the space between these walls. It comprises a metal foil resistance element 42, suitably slotted to provide a multiplicity of current paths, sandwiched between inner and outer layers of insulation similar to those of the embodiment shown in Figures 1 to 4. In this case, however, the inner layer 43 is thinner than the outer layer 44. Also, the outer layer 44- is spaced from the outer wall 41 by a layer 45 of heat-insulating material, such as glass cloth, bonded to the inner surface of the outer wall. These measures ensure that there is a considerably larger heat transfer from the resistance element 42 to the inner wall 40 than to the outer wall 41. This heat transfer is efiected through a liquid filling, up to the level 46. Although the filling must be fluid at the heating temperature, it is preferably a jelly at room temperatures.

The current for the resistance element is taken in through leads 47 contained in an internally insulated lug 48, the walls of which are preferably integral with or effectively secured to the walls 40 and 41 of the jacket. The leads enter a terminal block 49 to the pins 50 of which a standard socket can be connected. Preferably the terminal block 49 is also suitable for connection to a detachable lifting handle.

The vessel is provided with insulating feet 51.

The vessel can be used on its own, for heating its contents, or as an immersion heater for use in a larger utensil such as the dish 52. Securing clips 53 and 54 may be provided. For use with deeper utensils, it would be necessary to provide a longer lug 48.

For use as a booster heater in a liquid-filled radiator the heating device may be arranged in a form suitable for insertion into a liquid space of the radiator.

What I claim as my invention and desire to secure by Letters Patent is:

1. An electric resistance heating device comprising a metal foil forming a resistance element of large surface area to cross-sectional area ratio, a jacket enclosing said resistance element in close proximity therewith, at least part of the walls of said jacket constituting a heat radiating surface, liquid permeable insulation layers fitted between said resistance element and said jacket to prevent direct contact between the resistance element and the jacket, and an electrically insulating liquid substantially filling said jacket for transmitting by thermal conduction heat from the resistance element to said surface through the insulation layers.

2. An electric resistance heating device according to claim 1 in which said resistance element comprises a pattern of conductors formed of the metal foil.

3. An electric resistance heating device according to claim 2 in which said resistance element comprises at least one strip of metal foil divided into a plurality of conductive paths by slits.

4. An electric resistance heating device according to claim 3 in which the length of each of said slits is short in relation to the width of said strip, and said slits are disposed in rows with the slits in alternate rows arranged in staggered relationship.

5. An electric resistance heating device according to claim 1 in which said layers of liquid-permeable insulating material are sewn to said metal foil.

6. An electric resistance heating device as claimed in claim 1 in which an elastic anchorage is secured to at least one extremity of said insulation layers for holding said resistance element in tension and the insulation layers in firm contact with said heat radiating surface.

7. An electric resistance heating device according to claim 1 in which said jacket has a main heating surface and another surface, and the thickness of the layer between said main heating surface and said resistance element is less than the thickness of the layer between said resistance element and the other surface of said jacket.

8. An electric resistance heating device according to claim 1 in which said jacket has a main heating surface and another surface, and in which there is a layer of heat insulation on the inside of said other surface.

9. An electric resistance heating device according to claim 1 in which said jacket has a main heating surface and another surface, said other surface having a double wall enclosing a dead air space.

10. An electric resistance heating device according to claim 1 in which said liquid filling is free-flowing at the heating temperatures but is highly viscous at room temperature.

11. An electric resistance heating device according to claim 1 in which said liquid filling is free-flowing at the heating temperature but is a gel at room temperature.

12. A vessel for heating liquids comprising a jacket having a common wall with said vessel, a metal foil resistance element enclosed in said jacket, layers of liquidperrneable insulating material substantially filling the spaces between said metal foil element and said jacket, and a liquid which is an electrical insulator substantially filling said jacket for transmitting, by thermal conduction, heat from the resistance element to the jacket through said layers.

13. A domestic heating utensil comprising an inner dish-like wall, an outer wall spaced from said inner wall, a metal foil resistance element enclosed within the space bounded by said walls, layers of liquid-permeable insulating material substantially filling the spaces between said resistance element and said walls, and a liquid which is an electrical insulator substantially filling said spaces for transmitting, by thermal conduction, heat from the resistance element to the jacket through said layers.

References Cited in the file of this patent UNITED STATES PATENTS 1,028,107 Hadaway June 4, 1912 1,065,015 Yournans June 17, 1913 1,332,030 Collinson Feb. 14, 1920 2,000,438 Dougherty May 7, 1935 2,034,800 Dougherty Mar. 24, 1936 2,060,795 Burke Nov. 17, 1936 2,329,766 Jacobsen Sept. 21, 1943 2,504,146 Mossin Apr. 18, 1950 2,643,320 Pfenninger, Jr June 23, 1953 

